Browsing by Author "Rösch, Frohmut"

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Open Access
Atomistic dynamics of crack propagation in complex metallic alloys
(2008) Rösch, Frohmut; Trebin, Hans-Rainer (Prof. Dr.)
The failure of solid matter is familiar to us from everyday life. A broken dish or a shattered glass is annoying. The breakage of a ship or an airplane hull, however, can cost human lives. Nevertheless, insight into the fundamental mechanisms leading to fracture has been gained only within the last couple of years. One reason for this is that fracture is a multi-scale phenomenon. A macroscopic external strain is directed to the crack tip, where it breaks atomic bonds. Thus, to understand fracture mechanisms in macroscopic devices, one also has to know the processes on the atomic scale. These are hardly accessible by experiments, in particular when dynamic aspects are the center of interest. They are also not included in the classical elastic theory of fracture. Computer simulations have proven to be a useful tool to examine fracture processes on an atomic level. In molecular dynamics simulations the trajectories of the atoms are calculated by integration of Newtons equations of motion. Numerical experiments with simple crystal structures and model interactions reveal phenomena that are related to the discrete nature of matter and therefore cannot be explained by continuum theories. In more complex systems the mechanisms are not yet clear. In the current work two extreme cases of complex metallic alloys are investigated. The model quasicrystal is built-up mainly from atomic clusters. Due to the aperiodic long-range order, no unit cell exists. A major building block of the quasicrystal can also be used to obtain a periodic Friauf-Laves compound. The Friauf-Laves phases are topologically close-packed binary structures and form a huge class of intermetallic compounds. Among them are many candidates for high-temperature use. However, their brittleness at low and ambient temperature limits applications. Molecular dynamics simulations are performed to examine this brittle behavior at low temperature. First, model potentials are applied to qualitatively probe the influence of the underlying structure on crack propagation. In a second step, a specific Friauf-Laves compound is chosen. Interatomic potentials are constructed, which reproduce quantities obtained by quantum-mechanical calculations. Systematic fracture simulations then are performed.
Open Access
Numerische Studien zur Rissausbreitung in dreidimensionalen komplexen Kristallstrukturen
(2003) Rösch, Frohmut
In dieser Arbeit werden numerische Experimente zur Rissausbreitung im Modus I in einem dreidimensionalen ikosaedrischen Modellquasikristall präsentiert. Quasikristalle besitzen wohldefinierte atomare Ebenen und weisen atomare Cluster als elementare Bausteine auf. Der Einfluss dieser strukturellen Eigenschaften auf den dynamischen Bruch wird untersucht. Zu diesem Zweck werden verschieden orientierte Proben mit atomar scharfen Rissen versehen und anschließend durch lineare Skalierung des Verschiebungsfeldes belastet. Die Antwort des Systems wird dann mit Hilfe molekulardynamischer Simulationen erfasst. Sprödes Bruchverhalten ohne Anzeichen von Versetzungsemission wird beobachtet. Die Bruchoberflächen sind auf Clusterskala rau. Die Cluster werden jedoch nicht strikt vom Riss umlaufen, sondern zu einem gewissen Grad angeschnitten. Verglichen mit dem ebenen Anriss werden die Cluster allerdings weniger häufig durchtrennt. Demzufolge kann die Rauigkeit den Clustern zugeordnet werden, wohingegen die konstante mittlere Höhe der Bruchoberflächen die Ebenenstruktur des Quasikristalles widerspiegelt.